This invention relates to temporally stretch and compress laser pulses and particularly for the application of amplifying sub-picosecond laser pulses (less than 1 ps) to an energy level greater than 100 micro joules (J) per pulse.
An optical pulse stretcher-compressor is a key element in a chirped-pulse or regenerative laser amplifier for amplifying sub-picosecond laser pulses to an energy level greater than 100 micro joule per pulse. In this application, the stretcher is used to lengthen the optical pulses before the amplification, and the compressor is used to resume the original pulse duration after the amplification. In this way, the peak power inside the amplifier cavity can be kept low enough to avoid any damage to the optical elements and avoid nonlinear distortion on the pulse shape and beam profile.
Using a pair of diffraction gratings to compress optical pulses was proposed by Treacy 1 in 1969 and then by Martinez 2 in 1987. Grating-based optical pulse stretcher and compressor were first investigated experimentally by Pessot, Me., and Mourou 3 in 1987.
In the early design of Pessot etc. 3, four identical diffraction gratings were used. Two of the gratings were used in the stretcher to lengthen an ultrashort laser pulse by introducing a positive group velocity dispersion to the pulse. The other two gratings were used in the compressor to compensate precisely the phase modulation in the stretched pulse by introducing a negative group velocity dispersion.
Recent advances in self mode-locked Titanium:sapphire lasers and regenerative Titanium:sapphire amplifiers have led to a rapid development of laser pulse stretcher-compressors. Modified designs of the Pessot's stretcher-compressor 3 are commercially available and have four gratings 4, three gratings 5,6 or two gratings 7. Although the basic mechanism of phase modulation remains the same, these new designs greatly simplify the structure of the instrument and reduce the difficulty in alignment.
However, a major problem remains in all of those multiple-grating stretcher-compressors, namely, all of the gratings require precise readjustment when a change in the laser wavelength is required. These adjustments are extremely inconvenient when frequent changes in laser wavelength is required. Besides, strictly matched grating pairs are required in the stretcher and compressor in order to obtain required good beam profiles and appropriate pulse duration.
There is a continuing need for improvements in apparatus and methods for laser pulse stretcher-compressors.